Toluidine blue is a basic dye that binds nucleic acids, but preferentially stains RNA. When Toluidine blue stains structures blue, the staining is said to be orthochromatic. In some circumstances, the color of the bound dye is different from the free dye. Toluidine blue can form polymeric aggregates when exposed to high concentrations of polyanions that shifts its color from blue to red or purple. This behavior is called metachromasia.
Toluidine blue is often used to identify mast cells by virtue of heparin (which is very polyanionic) in their secretion granules.
Mast cells are scattered in the connective tissue surrounding this lymph node.